Method of growing alpha-alumina single crystal ribbons



Jan. 14, 1969 J- J. SHYNE ETAL Original Filed June 10. 19 64 FEE-GROWTHSTAGE DRY H Y D R OCiEN HIGH FLOW RATE ELEVATED TEMP.

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II/VVENTORS JAMES J. SHY/VE JOHN M M/LEWSK/ United States Patent3,421,851 METHOD OF GROWING ALPHA-ALUMINA SINGLE CRYSTAL RIBBONS JamesJ. Shyne, Caldwell, and John V. Milewski, Saddle Brook, N.J., assignorsto General Technologies Corporation, Reston, Va., a corporation ofDelaware Continuation of application Ser. No. 373,982, June 10,

1964. This application Mar. 26, 1965, Ser. No. 443,153 US. Cl. 23142 12Claims Int. Cl. C01f 7/42 ABSTRACT OF THE DISCLOSURE A new ribbon formof single crystal alpha-alumina is described. This form is characterizedby advantageous crystallographic and geometrical properties. An improvedprocess is described for making such a product in high yield during ashort growth period. The process involves periodically increasing thewater concentration of hydrogen gas passed over a melt of aluminumduring growth of the ribbons. A ceramic receptacle of predeterminedcomposition and made under certain firing conditions is described forholding the aluminum melt during the process.

This invention relates to alpha-alumina single crystals, and moreparticularly, to a method of making ribbons of such material in highyield.

This application is a continuation of application Ser. No. 373,982,filed June 10, 1964, by James J. Shyne and John V. Milewski and nowabandoned.

Single crystals of alpha-alumina, otherwise known as sapphire, in theform of extremely fine fibers, or whiskers, are known in the art topossess unusually high strengths. For example, such sapphire whiskershave been shown to possess tensile strengths of from two to four millionp.s.i. This property makes the material extremely desirable forreinforcing structural materials, such as metal and plastic.Alpha-alumina whiskers have been grown by passing hydrogen gascontaining a small amount of water over molten aluminum to form avolatile lower aluminum oxide. Thereafter the vaporous aluminum oxide iscondensed at a lower temperature onto a substrate whereupon itdisproportionates into aluminum and single crystal fibers ofalpha-alumina.

At the conclusion of the run, a portion of the aluminum charge also isoxidized to bulk alumina. This by-product material generally is presentin the form of cluster balls over the residue of the aluminum charge.The desired fibrous alpha-alumina is found among these cluster balls andmust be separated therefrom. Not only is the yield of fibrous materialin a given batch small, but the isolatable amount is even smaller. Thus,lack of available material has retarded commercial utilization ofalpha-alumina whiskers.

Accordingly, it is an object of the present invention to provide amethod of making alpha-alumina single crystals in high yield.

Another object of this invention is to provide a method of growingsingle crystal alpha-alumina in the form of ribbons substantially out ofcontact with other growth products.

These and other objects of the invention will be made apparent from thefollowing more particular description of the invention.

In general, the method of the present invention includes first providingan aluminum source material in a refractory receptacle, preferablycomposed of a fired-intimate mixture of particles comprising alumina andfinely-divided aluminum metal. The aluminum content is usually be tweenabout 1 to 20% by weight of the receptacle. The receptacle containingthe aluminum charge then is placed in a furnace at a temperature betweenabout 2200 and 3500 F., whereupon a melt of aluminum is produced.Thereupon the melt is contacted with a predetermined concentration ofwater vapor in a hydrogen gas atmosphere. The Water concentration andflow rate of the gas is adjusted in a predetermined manner duringvarious stages of the run. Alpha-alumina single crystals in ribbon formthen is de' posited in high yield on the walls and edges of thereceptacle. Since the ribbons are formed substantially out of contactwith by-products formed during the run, it can be readily provided inisolated form.

In accordance with the invention, the course of the growth of thealpha-alumina ribbons is controlled by a number of parameters,particularly the composition of the receptacle, the amount of moisturein the hydrogen gas over the aluminum charge, the rate of flow of thehydrogen gas stream, and the temperature of the reactants, at variousstages during the run. In this manner the reactants are developed at auniform rate, and thus build-up of the ribbons takes place uniformly.

The apparatus used in the process is of standard design. This includes atubular electric-resistance furnace to heat the charge to the moltenstage, and a gas train system to supply the desired hydrogen gas stream.

In the accompanying drawings:

FIGURE 1 is a flow sheet to illustrate the process steps according to apreferred practice of the invention.

FIGURE 2 shows the formation of the desired alphaalumina ribbons on theedges of the receptacle.

Referring now to the flow sheet of FIGURE 1, the process of theinvention is illustrated wherein improved yields of alpha-aluminaribbons are obtained. The run proceeds in three stages, designated thepre-growth, growth, and final growth stages. During the pregro'wthstage, the aluminum charge is wetted into the ceramic receptacle. Noribbons grow during this stage. The hydrogen over the aluminum charge iskept as dry as possible in the pregrowth stage, suitably containing nohigher than 50 ppm. At the end of the pro-growth stage an initial burstof vaporous aluminum oxide is observed, and the stage of nucleation andgrowth of the ribbons commences. At this point, a substantial amount ofwater is added to the atmosphere above the melt, suitable up to amaximum of 40,000 p.p.m. The water preferably is added incrementallywhile the ribbons are growing. A maximum water concentration is presentover the melt at the end stage of the run, referred to herein as thefinal growth stage. Preferably the water concentration is about 300 ppm.at the beginning of the growth stage and about 3000 ppm. during thefinal growth stage.

The pre-growth stage at the start of the run, usually takes about 10minutes. During this stage the dry gas stream is maintained at a highrate of flow, generally between about 0.01 to 0.06 cu. ft./sec., andoptically at about 0.03 cu. ft./sec. During ribbon growth, however, thehydrogen flow rate is substantially reduced relative to the initial drygas flow rate. Preferably the rate for the early stages of growth is inthe range of 0.001 to 0.03 cu. ft./sec., with 0.001 cu. ft./sec. beingconsidered optimum. For the final stage of growth, a flow rate of 0.0001cu. ft./ sec. is preferred. The fiow rates given are relative to thedimensions of the apparatus used, which are described in detail in theexamples.

The temperature of the melt may be held constant during the run, as forexample, between about 2200-3500 F. However, for a high yield of ribbonproduct, it is preferable that the melt be heated at a steadilyincreasing temperature during the growth and final growth stages,preferably from about 2500" to about 3100 F., during a run of about anhour.

The yield and collectability of the desired fibers also is increasedwhen the receptacle is of a predetermined composition. While aluminaitself may be used, it is a feature of the invention that the receptacleis constructed of a fired-intimate mixture of particles comprisingalumina and finelydivided aluminum metal. The aluminum particles usuallyare present in an amount comprising between about 1-20% by weight of thereceptacle. Generally the finer the particle size of the aluminum, thelower is the amount of aluminum required in the receptacle composition.For example, at a particle size of 200 mesh, an aluminum content of2.5-7.5% is preferred.

Usually the ceramic material used to form the receptacle includes, inaddition to alumina, metal oxides such as silica and iron oxide, and toa lesser degree, titania, chromium oxide, zirconium oxide or cobaltoxide. These oxides appear to function beneficially in the process.

The manner of preparing the receptacle also is an important factor inincreasing the yield of ribbons. In accordance with a preferredembodiment of the invention, the finely-divided alumina, or an aluminacomposition, and the aluminum powder, are mixed thoroughly and fired atan elevated temperature, preferably between about 20002500 F., andoptimally at about 2200 F.

The presence of aluminum metal in intimate contact with alumina in thereceptacle for the aluminum charge is important to the process becausesuch a receptacle enables the production of vaporous aluminum oxide at amore controlled rate than with an aluminum receptacle, thus providing asustained growth step at a uniform rate.

In accordance with another feature of the invention, a small amount ofalumina material is admixed with the aluminum charge, suitable betweenabout -50% by weight of the charge, and preferably about 20% by weight,to still further improve the yield of ribbons.

Referring now to FIGURE 2, there is schematically illustrated theformation of the alpha-alumina single crystal ribbons on the receptablein accordance with the present invention. A receptacle 1 carries analuminum charge 2 therein, which has been partially converted to thedesired alpha-alumina product 3. The product forms as ribbons on theedges and sides of the receptacle. Alumina cluster balls 4 are obtainedas a by-product. Only a small amount of fibrous material 5 is formed onand within the cluster balls themselves.

A typical alpha-alumina single crystal product in accordance with thepresent invention is a ribbon of rhombohedral cross-section. The ribbonhas a width to thickness ratio of between about 1:1 to 12:1, and lengthfrom to 25,000 times the width. Generally the length is from Example 1An alumina boat having the dimensions 5 x 17 x 2 inches, and weighingabout 3 lbs. 4 oz. is charged with 2.5 pounds of aluminum pellets whichare spread evenly on the bottom of the boat. The boat and aluminumcharge then is inserted into a furnace having a cross-sectional openarea of 20 square inches. The temperature of the furnace is set at 2775F. Purified hydrogen containing less than 20 p.p.m. of water vapor thenis admitted into the furnace over the charge at a flow rate of 0.01 cu.ft./sec. for a period of about ten minutes. Thereupon a vaporoussubstance is produced from the charge. At this point 300 p.p.m. of wateris added to the hydrogen gas stream and the flow rate is decreased to0.001 cu. ft./sec. Ribbon formation then proceeds on the edges of theboat. After about twenty to thirty minutes, the water content of thehydrogen is increased to 3000 p.p.m. and the flow rate is decreased to0.0001 cu. ft./sec. The total growth period is about an hour. The boatthen is removed from the furnace, cooled to room temperature, and theribbon product is isolated. The ribbons are alpha-alumina singlecrystals having a rhombohedral cross-section. A yield of about 0.2 g. ofribbon material is collected. The width of the ribbons average between10-100 microns; the length about 2 inches; and the thickness about 1-25microns.

Example 2 A boat for the aluminum charge is prepared by mixing 3 lbs. 2oz. of alumina powder and 0.16 lb. of 200 mesh aluminum powder (5% byweight of aluminum) in a slipcasting mold. The green ceramic then isfired in air at 2200 F. for about a half-hour. The boat receptacle thusprepared is used in place of the alumina boat in Example 1 and theprocess of Example 1 is carried out in a similar manner with an aluminumcharge. Using a boat of this composition, the yield of alpha-aluminaribbons is increased thereby to 2 g. The ribbons also are found moreabundantly on the sides and walls of the boat, as represented in FIGURE2.

Example 3 A mixture of 3 lbs. 2 oz. of ceramic powder material sold byNorton and Co. by designation 1162 having the following composition:parts by weight alumina, 15 parts by weight silica, 5 parts by weighttitania, 2.5 parts by weight ferric oxide and 2.5 parts by weight ofother metallic oxides, and 0.16 lb. of 200 mesh aluminum powder (5% byweight of aluminum) is prepared in a slip-casting mold. The greenceramic then is fired in air at 2200 F. for about a half-hour. The boatthus prepared is used in the process described in detail in Example -I.The yield of alpha-alumina ribbons is increased thereby to about 2.5 g.

Example 4 Into the boat receptacle prepared in the manner described inExample 3 is placed a charge of 2.5 pounds of aluminum pellets. The boatand aluminum charge is placed in a furnace heated at 2775 F. Purifiedhydrogen containing less than 15 p.p.m. of water vapor then is admittedat a fiow rate of 0.03 cu. ft./sec. for a period of about ten minutes.Then the water content of the hydrogen gas is increased to 350 p.p.m.and the fiow rate is decreased to 0.001 cu. ft./sec. Concurrently thetemperature of the furnace is increased, finally reaching a maximumtemperature of 3100 F. at the end of the run. Growth of alpha-aluminaribbons occurs on the sides and walls of the boat. After about 20minutes of growth, the water content of the hydrogen gas is increased toabout 3000 p.p.m., and the flow rate is decreased to 0.0001 cu. ft./sec. After a. total period of growth of about an hour, the ribbons arecollected. The yield is 8 g. of ribbons.

Example 5 A reaction charge consisting of 2.5 pounds of aluminum pelletsand 1.6 oz. of alumina chips are mixed together and placed in the boatreceptacle prepared according to the manner described in Example 3. Theprocess of Example 4 then is carried out. The ribbons thus obtained aresubstantially larger than those made in previous examples. Generallythese ribbons are about two inches in length, and some are as long asfour inches.

While the invention has been described with particular reference tocertain embodiments thereof, it will be understood that changes andmodifications may be made by those skilled in the art which do notdepart from the spirit of the invention. It is intended to be limitedonly by the appended claims.

What is claimed is:

1. In a process of growing single crystal alpha-alumina ribbons bypassing hydrogen and water vapor over a melt of aluminium in a ceramicreceptacle whereby a volatile lower aluminum oxide is formed, thereaftercondensed onto a substrate and disproportionated into aluminum andsingle crystal fibers of alpha-alumina, the improvement wherein a highyield of an elongated ribbon product is obtained in a short time,characterized by periodically increasing the water content of thehydrogen over the melt during the period of growth, starting fromessentially dry hydrogen and being increased substantially during therun to no higher than 40,000 p.p'.m.

2. The process according to claim 1 wherein the water content of thehydrogen is increased rapidly at the beginning of growth and thenincrementally while the ribbons are growing.

3. The process according to claim 1 wherein the water content of thehydrogen is increased from no higher than 50 ppm. to about 300 ppm. atthe beginning of growth, and then incrementally to about 3,000 ppm. atthe end of growth.

4. The process according to claim 1 wherein the temperature of the meltis increased periodically during the run.

5. The process according to claim 1 wherein the flow rate of thehydrogen initially is rapid, and then is substantially decreased duringthe end of the run.

6. The process according to claim 1 wherein the ceramic receptaclecontains alumina, silica, titania, iron oxide, chromium oxide, cobaltoxide or zirconium oxide.

7. The process according to claim 1 wherein the ceramic receptaclecontains at least silica, titania, iron oxide, chromium oxide, cobaltoxide or zirconium oxide.

8. The process according to claim 1 wherein the ceramic receptaclecontains between l2% by weight of finely-divided aluminum particles.

9. The process according to claim 8 wherein the ceramic receptacle ismade by firing its constituents in air at a temperature of about22002500 F.

10. The process according to claim 8 wherein the aluminum content of theceramic receptacle is about 25-75% by weight.

11. The process according to claim 1 wherein the aluminum melt is wettedinto the ceramic receptacle during a pre-growth stage of the processduring which no ribbons grow by passing hydrogen containing no more thanppm. water over said melt.

12. The process according to claim 11 wherein the hydrogen during thepre-growth stage is passed over the melt at a greater flow rate and themelt is maintained at a lower temperature than during growth of theribbons.

References Cited UNITED STATES PATENTS 2,741,822 4/ 1956 Udy 106- X3,077,380 2/ 1963 Wainer et al 23-142 3,011,870 12/ 1961 Webb et a12J3142 3,147,085 9/19 64 Gath 23142 X 3,298,842 1/1967 Senfert 106-65FOREIGN PATENTS 608,032 11/1960 Canada.

OSCAR R. VERTIZ, Primary Examiner.

HERBERT T. CARTER, Assistant Examiner.

